1. Field of the Invention
The present invention relates to a demodulation apparatus for digitally modulated waves which are utilized in satellite communications, satellite broadcasting, terrestrial communications, terrestrial broadcasting, and the like.
2. Description of the Related Art
Various communication systems are developed, as communication needs increase and communication techniques are developed. Among such communication systems, a system for transmitting video signals, audio signals, and other signals adopts a digitally modulating technique which is effective for improving the quality of transmission and frequency utilization efficiency.
Conventional terrestrial digital microwave communications utilize a multiple-value quadrature amplitude modulation (QAM) such as 16 QAM and 64 QAM, because such multiple-value QAM techniques can utilize frequencies with good efficiency. Conventional satellite communications utilize binary phase shift keying (BPSK) and quadriphase phase shift keying (QPSK), because the BPSK and QPSK techniques have good ,error rates of transmission codes.
In recent years, such digital transmission techniques have become often used in consumer systems such as mobile communications and advanced television (ATV). The digital transmission technique is regarded as a promising technique because of its high-quality signal transmission characteristics, its good frequency-utilization efficiency, and its superior compatibility with other media. In terms of consumer systems, the digital transmission technique has the following significant advantages. The circuit scale is small with a simple hardware configuration, the number of portions which require adjustment is small, the temperature drift is small, and the technique is suitably implemented in an integrated circuit (IC).
FIG. 6 is a block diagram showing a conventional demodulation circuit using a digital signal processing technique. A digitally modulated wave is input to an input terminal 30. The modulated wave is split and fed to an inphase detector 31 and a quadrature detector 32, respectively. A signal from a local oscillator 34 is input into the inphase detector 31 as a local-oscillator output with 0-degree phase shift. The signal from the local oscillator 34 is also input into the quadrature detector 32 as a local-oscillator output with 90-degree phase shift, after the phase of the signal from the local oscillator 34 is shifted by a 90-degree phase shifter 33. Each of the inphase detector 31 and the quadrature detector 32 converts the frequency of the received modulated wave signal into a base-band signal, by multiplying the modulated wave signal by the signal from the local oscillator 34. The inphase- and quadrature-detector outputs are input into analog low-pass filters 37 and 38 via buffer amplifiers 35 and 36, respectively. By the function of the analog low-pass filters 37 and 38, the high-frequency components of the detector outputs are removed. The outputs of the analog low-pass filters 37 and 38 are input into analog-to-digital (A/D) converters 41 and 42 via buffer amplifiers 39 and 40, respectively. Each of the A/D converters 41 and 42 samples the received signal in accordance with a sample clock from a sample-clock generator 43, and converts the signal into a digital signal. The rate of the sample clock is twice as high as that of the occupied bandwidth of the received modulated wave signal. In general, the rate of the sample clock is four times or more as high as that of the occupied bandwidth. The digitized detector outputs are input into digital channel filters 44 and 45 which have identical frequency transfer characteristics. In the digital channel filters 44 and 45, spectrum shaping is performed. These digital channel filters are implemented as filters for forming transmission characteristics which are required for intersymbol interference in a digital data transmission. Such a filter is often referred to as a roll-off filter. The filter is designed so as to exhibit desired characteristics when the characteristics are combined with the characteristics of a filter on the transmitter side. Specifically, the spectrum shaping is performed for the respective detector outputs so that the eye aperture ratios are increased at the outputs of the digital channel filters 44 and 45. The spectrum-shaped digital detector outputs are input into a demodulator 46. The demodulator 46 demodulates I-channel data and Q-channel data, and outputs the I-channel and Q-channel data from output terminals 47 and 48.
With the above-described conventional configuration, the quadrature detection processing of two channels, i.e., inphase and quadrature channels, from the input terminal 30 to the A/D converters 41 and 42 via the detectors 31 and 32, the buffer amplifiers 35 and 36, the low-pass filters 37 and 38, and the buffer amplifiers 39 and 40 is analog signal processing. Active devices (transistors, diodes, operational amplifiers) and other devices used in the analog signal processing are easily influenced by the temperature drift, the change with a time elapse, fluctuation of source voltage, and the like. Thus, the above-described conventional demodulation circuit is insufficient in stability. Since the analog signal processing necessitates respective circuits of inphase and quadrature channels, there exist various problems as a consumer demodulation apparatus in that the number of portions requiring initial adjustment is large, the production cost is high, and the configuration is not suitably implemented in an IC. A fundamental configuration for a digital detection processing type demodulation apparatus is disclosed in Japanese Laid-Open Patent Publication No. 59-207768, but the timing processing for I and Q signals is not disclosed therein.